Control of the read/write head position in a disk drive relative to track centerline on the disk is necessary to insure accuracy of the disk drive read and write operations. Over the years, many types of servo systems have been devised to detect and correct misalignment between the head and track centerline. One type of disk drive servo system is known as a sampled servo system. In such a closed-loop track-following system, servo information in the form of burst patterns is recorded in one or more servo sectors on each disk track. The servo information is read by the head during servo operations at each servo sector and used to generate position error signals as a function of the offset between the head and the disk track centerline. The position error signals are input to a microprocessor. The microprocessor in turn performs appropriate calculations with the position error signals and outputs servo compensation signals which control the disk drive head positioning mechanism to place the read/write heads over track centerline. The entire closed-loop arrangement is often characterized as the disk drive primary servo loop.
The predictable and periodic motion of a point at track centerline on the rotating disk surface relative to a fixed point on the disk drive base plate is known as the repeatable runout signature. Repeatable runout is caused by static offsets or perturbations of some sort in the mechanical portion of the disk drive "plant" consisting of the spindle motor, head positioning mechanism and related electronics. The position error signals generated for the primary servo loop at each servo sector contain the repeatable runout signature in combination with non-repeatable servo information, and represent the misalignment between the head and track centerline. When the magnitude of misalignment between the head and track centerline exceeds some maximum level, the primary servo loop may not be able to reduce the misalignment to within the tolerances required by the drive. It is thus often necessary to reduce the amount of repeatable runout using methods independent of the primary servo loop operation.
In order to reduce repeatable runout independent of the primary servo loop, some prior art disk drive servo control systems have utilized open-loop "static" feed-forward techniques to process position error signals received during servo operations and generate appropriate feed-forward corrections. Specifically, feed-forward correction is accomplished by detecting and measuring repeatable runout (using frequency selective filters based on discrete fourier transforms or the like) and then computing a feed-forward correction waveform which consists of a series of discrete feed-forward correction values, one for each servo sector, to be applied to the servo compensation signal. Specifically, at each servo sector the corresponding feed-forward correction value, which is proportional to measured repeatable runout, is added algebraically to the conventional primary servo loop compensation signal at that servo sector to account for repeatable runout present in the mechanical plant of the disk drive.
Servo systems which rely on so-called static feed-forward techniques often sample and average the position error signals across a number of disk revolutions to obtain an accurate measurement of the repeatable runout before computations to correct for repeatable runout can be performed. Typically, in a removable cartridge disk drive the feed-forward correction waveform is determined once at each cartridge insertion and may thereafter be redetermined only when some major event (e.g., power interruption or excessive shock to the drive) occurs requiring recalibration.
Any relative movement between the disk surface and the head positioning mechanism (a rigid structure attached to the disk drive base plate) results in a change in repeatable runout signature. This is a problem for "static" feed-forward techniques because the recalibration required to account for changes in repeatable runout impacts on drive performance. Complicating matters, as the disk drive art advances, track density, measured in tracks per inch (TPI), continues to increase while the mechanical plant is packaged into smaller and smaller form factors. Mechanical forces, for example friction and stiction associated with the spindle motor and head positioning motor ballbearings, become more pronounced in a relative sense, creating unacceptable instability in the plant characteristics. The non-linear perturbations caused by such forces, though extremely small in an absolute sense, are becoming so pervasive that servo systems utilizing static feed-forward techniques no longer provide adequate servo compensation.
In the frequency spectrum of the repeatable runout signature, the frequency components of interest for purposes of feed-forward correction normally lie at or below the rotational frequency of the disk, but under some conditions (e.g., unbalanced or warped disks or unfavorable mounting conditions caused by mounting warpage, spindle dynamics or the like) may also occur at frequencies above the disk rotational frequency. The "static" feed-forward algorithm operates to reduce the level of repeatable runout occurring in the position error signals at all frequencies of interest.
However, because the "static" feed-forward correction waveform is updated on an infrequent basis, conventional feed-forward correction techniques cannot keep track of mechanical plant changes and attendant position errors which arise over time while the disk drive is in operation. Hence, it would be highly desirable to provide a means for dynamically updating the feed-forward correction waveform otherwise applied to the compensation signal generated in the primary servo of the disk drive, which means takes account of any change in the character of the repeatable runout frequency components.
It is accordingly an object of the present invention to minimize the position error at the repeatable runout frequencies, whereby the read/write head remains over the track centerline to within the required tolerances.
It is a further object of the present invention to supply an adaptive feed-forward algorithm which, when employed in conjunction with a primary closed-loop or track-following sampled servo system, has sufficient bandwidth to track servo system mechanical plant variations and corresponding changes in the repeatable runout signature at the frequency components of interest.
These and other objects of the present invention are achieved in a servo system which implements a novel adaptive feed-forward algorithm. The adaptive feed-forward algorithm monitors the repeatable runout left uncorrected by the primary track-following servo system and periodically updates a feed-forward correction waveform as appropriate. In a preferred embodiment of the present invention, repeatable runout is measured over a single disk revolution. Subsequent feed-forward correction values based upon these measurements are computed and applied over the next succeeding disk revolution. The measurement and computing processes are thereafter repeated in continuous cycles while the disk drive performs its read and write duties. The rate at which the measurement/computing cycles occur and the gain at which the feed-forward corrections are accumulated directly determines the maximum overall adaptive feed-forward bandwidth achievable. In the case of a two revolution cycle time, the maximum adaptive bandwidth is one-half the rotational frequency of the disk.
The measurement of repeatable runout left uncorrected by the primary track-following servo loop is obtained by adding an adaptive feed-forward correction value from a current adaptive feed-forward correction waveform to the position error signal at each servo sector. A discrete fourier transform is performed on the resulting sum at each frequency of interest. Real and imaginary terms are accumulated over the sample interval for each sampled servo sector on the track. The accumulated real and imaginary terms are thereafter used to reconstruct a waveform correction corresponding to the repeatable runout at the frequencies of interest.
In a preferred embodiment of the present invention, the sample interval is a full revolution of the disk and the waveform correction is a pure sine wave at the primary runout frequency (i.e., the rotational frequency of the disk). The value of the waveform correction at each servo sector is applied, either directly or indirectly, to the primary servo loop compensation signal at each servo sector and simultaneously stored in a table as the current adaptive feed-forward correction value. The entire process, which essentially comprises a frequency-selective filter or filters at the frequencies of interest (e.g., a bandpass filter at runout frequency in the preferred embodiment), is thereafter repeated at some periodic interval.